Which control plane component is responsible for updating the node Ready condition if a node becomes unreachable?

The node controller, which is a part of the kube-controller-manager, is responsible for managing the lifecycle of nodes. It periodically checks the health of each node. If a node becomes unreachable (e.g., stops sending heartbeats), the node controller updates the node's status in the API Server, marking its Ready condition as Unknown and eventually False. It is also responsible for evicting pods from the unreachable node after a configured grace period.

A. The kube-proxy runs on each node to manage network rules; it does not monitor or update the health status of other nodes.
C. The kubectl is a command-line client used by users to interact with the cluster; it is not an automated control plane component.
D. The kube-apiserver exposes the API but relies on controllers, like the node controller, to actively monitor and update the state of cluster objects.

1. Kubernetes Documentation. (n.d.). kube-controller-manager. Kubernetes. Retrieved from https://kubernetes.io/docs/reference/command-line-tools-reference/kube-controller-manager/. (See section on "Node controller").

2. Kubernetes Documentation. (n.d.). Node Concepts. Kubernetes. Retrieved from https://kubernetes.io/docs/concepts/architecture/nodes/#node-controller. (The "Node Controller" section details its responsibility for node status).